A new method for measuring porous microbial barriers: Part II A closer look at ASTM international standard test method F2638

Background

Balloting on a new test method for use in the ranking of porous packaging materials used in sterile packaging applications by their ability to provide bacterial holdout was completed in July 2007 by ASTM International Committee F02 Flexible Barrier Packaging. As a result of this balloting, there is a new test method available for evaluating the barrier capabilities of porous materials intended for use in the packaging of terminally sterilized products. This new test method, ASTM F2638-07 Standard Test Method for Using Aerosol Filtration for Measuring the Performance of Porous Packaging Materials as a Surrogate Microbial Barrier, utilizes an aerosol of 1.0µm diameter polystyrene spheres to measure the filtration efficiency of porous materials. Unlike its predecessor, ASTM F1608-00(2004), Standard Test Method for Microbial Ranking of Porous Packaging Materials (Exposure Chamber Method), which uses actual spores of a nominal 1.0 µm size for testing and can take several days to produce results, the apparatus defined in the new ASTM F2638-07 test method can provide almost instantaneous test results.

Method Summary

A porous packaging test specimen is placed into a sample holder to create a filter between the challenge and filtrate aerosol streams. On the challenge side of the sample chamber an aerosol of particles is presented to the surface of the test specimen. Air flow is generated through the test specimen and laser particle counters are used to enumerate the particles in both the challenge and filtrate aerosol streams. Enumeration of particles in the challenge and filtrate streams can be performed sequentially using only one particle counter or concurrently using two particle counters. A percent penetration value can be calculated from the challenge and filtrate particle count data. Monitoring and plotting percent penetration vs. flow rate through the sample generates a typical filtration efficiency curve. The maximum percent penetration value and the flow rate at which it occurred is determined from the peak value of the curve.

Significance and Use

This new test method has been developed as a result of the research conducted by Air Dispersions Ltd. (ADL) of Manchester, UK, and funded by the Barrier Test Consortium Ltd. (BTC).

Members of the Barrier Test Consortium Ltd. (BTC)
• Amcor Flexibles (formerly Rexam Medical Packaging)
• Billerud (formerly Henry Cooke)
• DuPont Medical Packaging
• Kimberly-Clark
• Oliver Products
• Perfecseal, a Division of Bemis Corp.
• Westfield Medical Packaging

The results of this research have demonstrated that testing the barrier performance of porous packaging materials using microorganisms correlates with measuring the filtration efficiency of the materials. The new method does not require the use of microbiological methods and can be conducted in a rapid manner. The incumbent test method for measuring microbial barrier, ASTM F1608, challenges test specimens at only one flow rate, a rate which is considered by many to be unrealistically high. In contrast, the new method generates filtration efficiency data over a range of flow rates that are considered to be more representative of the environment encountered by sealed packages during normal handling and distribution cycles.

When measuring the filtration efficiency of a porous packaging material, a typical filtration efficiency curve is generated. Because the arc of the curve is dependent upon the characteristics of the individual test material, the appropriate way to compare materials is to use the parameter that measures the maximum penetration through the material, the flow rate at which the most particles pass through the sample.

Testing Apparatus

The main components which comprise the new system are:

• an aerosol generator, which uniformly introduces polystyrene particles into the challenge air stream at a desired concentration;
• the sample holder;
• a manometer for measuring pressure drop across the test specimen;
• one laser particle counter for enumerating the particles in both the challenge and filtrate aerosol streams;
• a means of recording the data from the particle counters and pressure manometer;

While this is the most economical apparatus for conducting the new test method, the use of only one particle counter requires the challenge and filtrate aerosol streams to be monitored alternately. This doubles the amount of time required to perform the testing. In addition, the use of only one counter means the value of the challenge counts must be calculated vs. measured during the interval when filtrate counts are being measured. This increases data reduction difficulty and time. The addition of a second particle counter allows for simultaneous measurement of the challenge and filtrate aerosol streams. This reduces both testing and data reduction time.

Some additional instrumentation in the form of mass flow meters and pressure transducers can make the test system more user friendly. While adding some cost, this additional instrumentation can provide real-time information regarding aerosol generator flow, filtrate aerosol flow, vacuum generator pressure and challenge vent pressure. Constant monitoring of these various system parameters can alert the operator to possible system malfunctions, which could generate erroneous data. In addition, the direct measurement of the filtrate aerosol flow rate practically eliminates the need for converting pressure drop across the sample to flow through the sample, further reducing data reduction difficulty and time.

Development of the Standard Test Method

The development of a standard test method was conducted in parallel with the work to “finetune” the test unit. ASTM International was selected as the standards organization to develop the test because it is a consensus standards organization; it has a well-defined protocol for determining precision and bias; and it has a plethora of standards commonly used in the medical device packaging community.

The draft test method was submitted to ASTM Committee F02.0 Flexible Barrier Packaging which is responsible for medical packaging materials and systems. The test method F2638-07, entitled Standard Test Method for Determining the Microbial Barrier of Porous Packaging Materials (Aerosol Filtration Method using Dual Particle Counters), was balloted three times at the subcommittee and main committee levels before being accepted.

With the successful balloting and publication completed, additional test units are planned to be built by other members of the healthcare industry and a full round robin study will follow to establish inter- and intra-laboratory reproducibility.

The standard test method will be submitted to ISO technical committee TC198 (Sterilization) /working group WG7 (Packaging), which is responsible for ISO standard ISO 11607-1:2006 Packaging for terminally sterilized medical devices - Part 1: Requirements for materials, sterile barrier systems and packaging systems. This committee was recently responsible for activities that harmonized the ISO 11607 standard with the CEN medical packaging standard EN 868 Part

Microbial Barrier Tests…Old vs. New

ASTM F1608-00 (2004) Standard Test Method for Microbial Ranking of Porous Packaging Materials (Exposure Chamber Method), or the log reduction value (LRV) test as it is commonly called, also produces an aerosol and challenges the test specimen by generating flow through the specimens. However, this test is conducted at only one flow rate -- 2.8 ℓ/min. -- and instead of polystyrene spheres, the LRV test is conducted with live organisms of nominal 1 µm size. Spores that penetrate the test specimens are collected for plating and enumeration on membrane filters. Subsequent to plating, enumeration samples are incubated for a minimum of 24 hours. Colony forming units (CFUs) and dilution factors, if any, are recorded for each sample. The LRV is then

calculated by comparing the logarithm of the number of spores passing through the sample (the number of colonies on the plate) with the logarithm of the microbial challenge (the number of spores in suspension above or upstream of the sample). The use of live organisms for ASTM F1608 means that care must be taken when handling specimens. The colonies are often difficult to maintain; they will sometimes die off or become “unhealthy,” resulting in questionable test results. To reduce the possibility of generating erroneous results due to contamination from normal bioburden, test specimens are typically sterilized prior to performing the test. There is also a need for constant decontamination of instruments and equipment. Thus, it is almost a requisite that this testing be conducted at a biological testing facility or a wet lab.

In addition to sterilization and set-up time, the time required to conduct this test includes 15 minutes of actual sample exposure to microbial challenge; time to perform dilutions, if necessary; time to prepare plates; a minimum of 24 hours to incubate; and time to enumerate the CFUs. Then, data reduction can begin. Therefore, typical turnaround time quoted by contract test labs (to conduct a test, generate the data and write a report) is three to four days. In addition, it is important to note that this test is generally perceived as a rather “noisy” test, which could be attributed to the dilution process and how the CFUs are counted.

The most critical shortcoming of the ASTM F1608 test is the flow rate used during the challenge process. The flow rate for this test is fixed at 2.8 ℓ/min, which generates a face velocity greater that 140 cm/min. when accounting for the sample area. This rate is Inertial impaction – occurs when a particle, as a result of its mass, deviates from the air stream flowing around a fiber and collides with it. The effectiveness of this method of capture is directly related to the mass of the particle and the speed of the air stream. The higher the velocity and the mass of the particle, the greater the chance of it colliding with a fiber.

unrealistically high when compared to theoretical and real flow rates that would be encountered by packaging materials during actual package distribution and handling, as shown in Table I. (At a flow rate of 2.8 ℓ/min., inertial impaction is the dominant filtration mechanism.) At the face velocity caused by this flow rate, spores cannot rapidly change direction with the air flowing through the sample. Their inertia tends to cause collisions with filter fibers, entrapping the spores. Because of this phenomenon, some porous materials tested via this method appear to provide a much better microbial barrier than they actually do when tested at flow rates approaching real-world conditions where the percent penetration is much higher.

The area of the ASTM F2638 test specimen is four times larger than the area of the sample required for the ASTM F1608 test. Testing the same amount of surface area using ASTM F2638 requires one fourth the number of replicates as compared to ASTM F1608. Testing a larger area per sample also helps reduce sample to sample variability therefore minimizing data scatter.

Using two particle counters, the ASTM F2638 test method requires only three minutes to obtain sufficient data for percent penetration calculation at a given flow rate or pressure differential. If one counter is used to alternately monitor challenge and filtrate particles, the testing time is doubled. Data can be generated at five different flow rates in fifteen minutes using a two counter system. This is the same exposure time required to generate data for only one flow rate when using the ASTM F1608 test method. When testing via ASTM F2638, data reduction can begin immediately upon completion of a test. The population numbers are known immediately for both sides of the sample, filter reduction can be calculated and expressed in LRV; there is no wait time to count the downstream side. Since there are no actual organisms or spores, there is no need for decontamination, culture plating or incubation. In addition, particle counters do the actual enumeration.

Unlike with ASTM F1608, there is no need for a biological test facility when using the ASTM F2638 test method. Therefore, this test can be set up and run almost anywhere. In addition to rapid sample turnaround, the new test method has the ability to rapidly test at multiple flow rates. These flow rates can be adjusted to imitate conditions encountered by packages during actual distribution and handling. Data have been generated demonstrating that the maximum penetration point of porous materials typically used for sterile packaging applications occurs at flow rates below those used in the ASTM F1608 test.

It has become apparent to many in the medical device industry that ASTM F2638 is a more rigorous and realistic test for measuring porous microbial barriers than ASTM F1608. More can be learned regarding the barrier performance of porous materials by testing via the new method. Figures I and II show data using both test methods. Figure I displays data obtained using the ASTM F2638 test method on several common porous sterile barrier system materials. Figure II displays the LRVs obtained by using the ASTM F1608 test method for the same materials.

The Importance of Flow Rates

Figure III displays data collected while performing ASTM F2638. Percent of penetration is plotted versus test sample face velocity. These are basically the same data displayed in Figure I. However, displayed with these data is the face velocity experienced by test samples during testing conducted in accordance with ASTM F1608. Note the extremely high face velocity used in the ASTM F1608 test. Refer to Table I and compare this face velocity with the typical velocity generated by air transport or routine handling. In contrast, the range for face velocity used in the ASTM F2638 test is much closer to real-world stresses listed in Table I. Notice in Figure I or III that the maximum penetration points or curve peaks all occur at face velocities less than 5 cm/min. The maximum penetration points occur at a face velocity that is practically the same as those generated by real life stresses. Knowing this fact raises the question, why test at face velocities more than 100 times greater (ASTM F1608) than what might be encountered under typical transportation and handling environments? The point of maximum penetration is where the microbial barrier properties of a porous material are challenged to their limit. Therefore, these are the flow rates or velocities most important to know when selecting a barrier material.

When reviewing flow rates or face velocities, it is important to consider the various conditions a package may be exposed to during its life cycle. Every package will encounter many stresses and levels of stress during its life cycle. However, these levels or face velocities are only part of the equation. Factors such as package volume also affect rate of pressure differential equilibration in a porous sterile barrier system. For example: a flat, two-dimensional package has very little air in the original configuration to evacuate and, as a result, very little air enters during the equilibration phase.

The surface area of the porous package material also affects face velocity. Less surface area for air exchange means greater face velocity during any equilibration process. Small patches or vents limit the space through which a sterile barrier system can vent or equilibrate. Therefore, the forces are stronger, causing an increase in the face velocity. Another variable to consider is package dimensions -- LxWxD. A long, thin flexible sterile barrier system, such as a chevron peel pouch, exhibits different equilibration rates than a rectangular formed film design or a square rigid blister.

The clinical usage of a sterile barrier system also subjects it to different stresses. For example: the pressure changes that occur to a flexible pouch when a nurse reaches into a box of small disposables, removes a handful and carries them in her pocket are very different from the pressure changes experienced by a rigid blister package opened in an operating room setting.

Conclusion

What does all of this mean to a medical device packaging engineer? The new ASTM F2638 test method is an initial step toward establishing criteria and determining appropriate microbial barrier requirements for sterile packages. Additional work related to the relationship between package design, package volume, secondary packaging, clinical usage, environmental stresses and maximum penetration rates of materials will move the industry one step closer to adopting a universal standard for microbial barrier. Although it will take time to generate the data required to shift the industry paradigm related to acceptable microbial barrier, the new ASTM F2638 test method gives packaging engineers a valuable new tool to rank various materials and begin questioning the appropriate level of microbial barrier for medical devices.

References

1. ISO 11607-1:2006 Packaging for terminally sterilized medical devices Part 1: Materials, sterile barrier systems and packaging systems.
2. ASTM International F1608-00(2004) Standard Test Method for Microbial Ranking of Porous Packaging Materials (Exposure Chamber Method).
3. ASTM F2638-07 Standard Test Method for Using Aerosol Filtration for Measuring the Performance of Porous Packaging Materials as a Surrogate Microbial Barrier,
4. Microbiological Barrier Testing of Porous Medical Packaging Materials, Alan Tallentire and Colin Sinclair at the Meeting of the Society of Plastics Engineers (Scandinavian Section), May 1994.

Paul F. Herman is a Nonwovens Application Consultant for DuPont Medical and Industrial Packaging in Richmond, VA. Curtis L. Larsen is a Packaging Consultant for DuPont Medical Packaging and Spartan Design Group, LLC located in Tonka Bay, MN.